Another peril can be added to the hazards of the innocent-looking computer keyboard. Not content with encouraging repetitive strain injury, the type-pads sometimes harbour more filth than the average loo seat and house millions of bacteria which can cause diarrhoea and vomiting, a study has shown.

A microbiologist carrying out research published today for Which? Computing magazine examined samples from 33 keyboards and found a variety of bugs including E coli and S aureus, which can cause skin infections and make people ill.

The scientist swabbed a loo seat and a toilet door handle in a typical London office for comparison. One of the keyboards in the experiment had to be removed from the office because it was five times dirtier than the lavatory seat and home to 150 times the acceptable limit of bacteria.

"[It] was increasing the risk of its user becoming ill," said the microbiologist, James Francis. "I haven't seen a reading like that in a very long time - it was off the scale."

Two more of the keyboards had "warning levels" of bacteria, while a further two showed high levels of coliform bacteria, which are associated with fecal matter.

The research showed that the chief culprit for contamination was people eating at their desks and dropping crumbs which lodge between keys and encourage the growth of millions of bacteria.

Other causes included poor personal hygiene, particularly people failing to wash their hands after using the toilet. Dust, which traps moisture, also provides ideal conditions for the growth of bacteria.

"These results are generally typical of offices up and down the country," said Francis.

Sarah Kidner, the editor of Which? Computing, said: "Most people don't give much thought to the grime that builds up on their PC, but if you don't clean your computer, you might as well eat your lunch off the toilet.

"Why not give your PC a spring clean? It's quite simple to do and could prevent your computer from becoming a health hazard."

The survey, in which more 4,000 people were questioned, found that 22% cleaned their keyboard and 27% their monitor on a monthly basis. Just over one in 10 said they never cleaned their keyboard, while 20% said they never cleaned their mouse.

The magazine also warned that headphones can ferry germs between people and transfer headlice.

Which? Computing recommends that computers be shut down and unplugged before being gently dusted with a soft, lint-free cloth and wiped with an approved computer cleaning fluid.

A vacuum cleaner can be used to clean vents, but contrary to internet myth, keyboards should not be put in the dishwasher, the magazine said.